This invention relates to a method for laminating two substrates together which does not require the use of a solvent. More particularly, this invention relates to a solvent-free laminating adhesive composition for use in flexible packaging lamination. Most particularly, the invention relates to the use of novel, solvent-free adhesive systems utilizing lightly epoxidized low molecular weight block copolymers of conjugated dienes.
Many adhesive compositions have been suggested in the art for bonding materials such as rubber, wood, plastic, paper, fabric and the like to each other or other materials. Various types of polymers have been suggested for use in adhesive recipes to accomplish these goals and certain polymers, such as block copolymers of conjugated dienes and vinyl aromatic hydrocarbons have proved very useful in adhesive applications, imparting very good adhesive and elastomeric properties to adhesives made with the polymers.
There has, however, been a problem in making laminating adhesives which are used to bond two substrates together, especially in flexible packaging applications. While the above polymers can be used to make laminating adhesives, they suffer from the property of having prohibitively high melt viscosities. Thus, the energy that would be required would be excessive for melt application of these adhesives. Further, the temperatures required to achieve the melt state in the above mentioned polymers are too high for many polymeric substrates used to construct laminates. As such, the above mentioned polymers are useful as laminating adhesives only when carried by a solvent. In addition, wholly hydrocarbon polymers do not possess polar chemical functionality which can enhance adhesion to polar substrates. The invention in U.S. Pat. No. 4,135,037 attempted to add polar functionality by extensively epoxidizing these polymers of conjugated dienes and vinyl aromatic hydrocarbons and then using them in adhesive compositions requiring rosin based tackifying resins. Better results were shown with these epoxidized polymers than with the unepoxidized precursor polymers. However, these relatively high molecular weight epoxidized polymers had to be applied at 203.degree. C. or required the use of large amounts of solvent in order to lower the viscosity of the polymer sufficiently so that it could be used in the adhesive application.
Thus, it can be seen that there is a need for a low viscosity adhesive which gives sufficient adhesion characteristics and which is capable of being used in the absence of a solvent to make it more useful in laminating adhesive applications. One of the criterion in this application is that the adhesive layer be very thin, especially when it is used in a flexible packaging construction. This is different from most pressure sensitive adhesive applications which can use relatively thick layers of the adhesive. The present invention provides such a laminating adhesive.